The present invention relates to an automatic multispeed transmission for use in vehicles, and more particularly to an automatic multispeed transmission which is adapted to improve the driveability of a vehicle such as a motorcycle on which it is mounted, when the vehicle undergoes large running resistance.
Vehicles, in particular, motorcycles having small displacements are generally equipped with automatic multispeed transmissions each having a plurality of speed change gears which are each provided with a centrifugal clutch simple in construction to facilitate the speed controlling operation of the vehicles and smooth the starting or accelerating operation of same. In such automatic transmissions, the centrifugal clutches each forming part of a speed change gear are each adapted to engage at a predetermined engaging rotational speed, whereby the transmission automatically performs speed change at a plurality of changing points corresponding to the predetermined engaging rotational speeds of the clutches. Preferably, an automatic transmission of this type should be designed such that the driver can drive the vehicle at any desired speed and with any desired driving force or torque in various conditions including starting, accelerating and slope-ascending.
However, a vehicle equipped with a transmission of this type has a disadvantage in driveability when it undergoes large running resistance, for instance, when ascending a steep slope.
For example, in an automatic three-speed transmission for motorcycles, which has three centrifugal clutches for low, medium and high speeds, wherein the engaging rotational speeds of the clutches for the medium and high speeds, are set to 10 km/hr. and 20 km/hr., respectively, as two speed changing points, the motorcycle is driven by a high or third speed gear due to engagement of the high speed clutch when it is running at a speed higher than 30 km/hr.
The reduction ratio of the high or third speed gear is set at a value smaller than that of the medium or second gear so that the engine speed is low relative to the vehicle during high speed running, that is, during the engagement of the high speed gear. Therefore, when the running resistance applied to the motorcycle increases, for instance, in ascending a steep slope at a speed of 30 km/hr. with the high speed gear engaging, the running speed can decrease due to insufficient driving force. That is, the driving force required for ascending the slope can be obtained only after the running speed has decreased below about 20 km/hr. to change the high speed clutch to the medium speed gear. If the driver nevertheless increases the vehicle speed by operating the accelerator so as to ascend the slope at a speed of 30 km/hr. or so, the transmission is automatically shifted up to the high speed gear at the changing point of 20 km/hr., resulting in a drop in the driving force. Therefore, to avoid a drop in the driving force, the driver is compelled to keep the running speed at a speed below 20 km/hr., with the medium speed gear engaging during ascending the slope.
As explained above, a vehicle equipped with an automatic multispeed transmission of the above mentioned type which effects automatic speed change at predetermined fixed changing points regardless of the running resistance could fail to provide a running speed and a driving force desired by the driver.